Study On Microstructure And Properties Of Welded Joints Of 1MnCrMoNi Alloy Steel

The offshore platform is an important working platform for the exploitation of offshore oil and gas resources,welding is an indispensable processing method in its production and manufacturing processes.In practical engineering applications,Marine steel is often welded with carbon steel,pipeline steel and the same kind of marine steel.However,due to the large difference in physical and chemical properties between dissimilar metals,the weld joints can cause problems such as non-uniform chemical composition and mechanical properties,affecting the quality of welded joints.In addition,most of marine steel is a high-strength,large-thickness material,which has brought great difficulties to the welding of the same type of offshore steel.In this paper,shield metal-arc welding（SMAW）and gas tungsten arc welding（GTAW）methods are used to weld 1MnCrMoNi alloy steel for marine engineering and Q235carbon steel,X70 pipeline steel and the same 1MnCrMoNi alloy steel.Analytical testing methods such as metallographic microscope,scanning electron microscope,X-ray diffractometer,microhardness tester,impact tester,tensile tester,electrochemical workstation and immersion test were used to analyze the microstructure and phase composition of the welded joint,mechanical properties and corrosion resistance in simulated seawater.The results show that the Widmanstatten Structure was detected in the Q235 heat affected zone of 1MnCrMoNi/Q235 joints under SMAW and GTAW conditions,and the acicular ferrite dominates the welded seam,and the heat affected zone of the 1MnCrMoNi alloy steel is proeutectoid ferrite and Lath martensite.The microstructure of X70 heat-affected zone in1MnCrMoNi/X70 joints is refined,the Widmanstatten Structure is present in the surface of the GTAW joint X70 heat-affected zone,the difference between the bottom layer and the surface layer of the welded seam is larger,the coarse-grained Widmanstatten Structure was detected on the surface of the GTAW joint welded seam,the microstructure of the heat affected zone of the1MnCrMoNi alloy steel is larger.In the 1MnCrMoNi/1MnCrMoNi joint,the M-A component is locally present in the heat affected zone of the 1MnCrMoNi alloy steel,the SMAW joint welded seam are favourably staggered acicular ferrites,the bottom layer of GTAW joint welded seam is fine acicular ferrite,and the surface layer is a mixture of coarse bulk ferrite and pearlite.Under the condition of SMAW and GTAW,the hardness fluctuation of each zone in the1MnCrMoNi/Q235 welded joint is small,the hardness of the fine grain zone and the welded seam of the 1MnCrMoNi/Q235 welded joint of GTAW joint is higher than that of the SMAW joint,and the remaining areas are basically the same.The hardness of each region of1MnCrMoNi/X70 welded joint is between the two base metals,the hardness of welded seam and coarse grain region of 1MnCrMoNi alloy steel of the SMAW welded joint are higher than that of the GTAW welded joint;The hardness of the heat affected zone of1MnCrMoNi/1MnCrMoNi welded joints is not much different from that of the base metal and the welded seam hardness is much higher than that of the base metal and the heat affected zone.Under SMAW and GTAW conditions,the tensile strength of 1MnCrMoNi/Q235 welded joint is higher than that of the lower strength Q235 base metal,1MnCrMoNi/X70 welded joint tensile strength is equal to X70 base material,the tensile strength of 1MnCrMoNi/1MnCrMoNi welded joints is higher than that of 1MnCrMoNi alloy steels.Among them,the tensile fracture position of the three kinds of welded joints is in the heat affected zone,and the performance totally accord with the requirements of practical engineering applications.Under SMAW and GTAW conditions,the welded seam and the heat affected zone of 1MnCrMoNi/Q235 and1MnCrMoNi/X70 welded joints have good impact toughness and meet the application requirements,while the welded seam and heat affected zone of 1MnCrMoNi/1MnCrMoNi welded joint have large differences in impact toughness,but these all are greater than the standard value at the test temperature,the toughness is qualified.Electrochemical test results show that under the conditions of SMAW and GTAW,the corrosionpropensitiesof1MnCrMoNi/Q235weldedjointsare:Q235-HAZ>WM>1MnCrMoNi-HAZ,and the corrosion resistance of the welded joints of1MnCrMoNi/X70 and 1MnCrMoNi/1MnCrMoNi is almost the same.The results of immersion test show that the joint corrosion rate under SMAW of 1MnCrMoNi/Q235,1MnCrMoNi/X70and 1MnCrMoNi/1MnCrMoNi was slightly greater than that of GTAW.The morphology of Q235-HAZ and welded seam corrosion products of 1MnCrMoNi/Q235 joints are wrinkled and flaky,and the components areγ-FeOOH andα-Fe₂O₃;The morphology of the 1MnCrMoNi-HAZ corrosion product is a bat-like structure with the composition ofα-FeOOH.Both the1MnCrMoNi/X70 welded joint X70-HAZ and welded seam corrosion products are cotton balls ofα-FeOOH,1MnCrMoNi-HAZ corrosion products are rod-shapedα-Fe₂O₃ andγ-FeOOH.The corrosion products of 1MnCrMoNi/1MnCrMoNi welded joints 1MnCrMoNi-HAZ show a relatively dense composition ofγ-FeOOH andα-Fe₂O₃,while the welded seam corrosion products exterior that there is a large turtle crack and the composition wasα-FeOOH.Under the welding process conditions of this article,the three welded joints of1MnCrMoNi/Q235,1MnCrMoNi/X70 and 1MnCrMoNi/1MnCrMoNi is show high-quality,which can be a theoretical reference for the application of 1MnCrMoNi alloy steel in offshore engineering.